1. Field of the Invention
This invention relates to a new process for the preparation of oligourethanes containing free isocyanate groups which are soluble in aromatic solvents, to the oligourethanes with free isocyanate groups obtainable by this process and their use as binders or binder components in moisture hardening one-component lacquers.
2. Description of the Prior Art
Isocyanate prepolymers used as binders for moisture hardening one-component lacquers are well known in principle and general descriptions of these prepolymers are given, for example, in H. Kittel: Lehrbuch der Lacke and Beschichtungen, 1973, Volume I, part 2, page 572 et seq. They are generally prepared from low molecular weight polyols and macropolyols which are reacted with diisocyanates in proportions corresponding to a NCO/OH ratio of 1.4 to 2.0.
The solvents used in the state of the art for moisture hardening PUR lacquers are generally polar solvents such as esters of acetic acid, e.g. ethyl acetate or ethyl glycol acetate, optionally mixed with solvents which are less polar, such as hydrocarbons. The exclusive use of hydrocarbons as solvents is generally not possible because the solutions become very cloudy when left to stand or when prepared by diluting concentrated solutions to the working concentration. This renders them unsuitable for use.
The use of polar solvents, in particular ester-based solvents, in one-component polyurethane lacquers according to the state of the art has the further disadvantage that polar solvents, e.g. the acetic acid esters predominantly used, are retained in small quantities in the cured lacquer film for a long time. When these lacquers are used externally and therefore exposed to weathering, these esters undergo hydrolysis. The acetic acid then formed has two very disadvantageous effects: (1) it catalyzes the degradation of the lacquer films and (2) in lacquers applied to metal it reduces the protection against corrosion provided by the lacquer. Both of these factors reduce the durability of the lacquer coat.
It was therefore an object of the present invention to provide one-component lacquer binders which would form clear solutions in comparatively non-polar aromatic hydrocarbons and which could be further diluted to the required working concentration with apolar solvents such as aromatic or aliphatic hydrocarbons.
This problem was able to be solved by providing the process according to the invention described below for the preparation of oligourethanes containing free isocyanate groups and soluble in aromatic solvents. In this process, organic diisocyanates are reacted with varying quantities of macropolyols in the molecular weight range of from 500 to 10,000 and 2,2,4-trimethylpentane diol-(1,3) or the hydroxyl-containing derivatives thereof described below and optionally other low molecular weight polyols.
The preparation of straight chain, high molecular weight polyurethanes soluble in aromatic solvents by a process using 2,2,4-trimethyl pentane diol-(1,3) as chain lengthening agent has already been disclosed in DE-OS No. 3,417,475, but it could not be expected that the use of this diol would also provide the possibility of obtaining branched chain, comparatively low molecular weight oligourethanes with free isocyanate groups in a form that is soluble in aromatic solvents since straight chain, high molecular weight polyurethanes and comparatively low molecular weight branched chain isocyanate prepolymers belong to different classes of compounds with completely different structures.
Low molecular weight, urethane-modified diisocyanates based on 2,2,4-trimethyl-pentane diol-(1,3) and the hydroxyl-containing derivatives thereof are described in an earlier proposal by the present applicants (German patent application No. P35 02 265.5), but no macropolyols of the type described below as starting component (b) according to the invention are used in the preparation of these modified polyisocyanates.